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  1. #1
    Veteran Member Three Rings killerkali's Avatar
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    Jan 01 2011
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    Voltage drop after warming up, not the alternator

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    Trying to diagnose an issue that has developed over the last year. On a cold start I get a good output from the alternator(~14.3), but once the car has fully warmed up the voltage drops to around 13.5v. Initially I replaced my Alt with a used on, and had the same issue. Finally put a new OEM bosch in and although it seemed to help(voltage only will go down to 13.7) I still have this issue. Battery is relocated to the trunk with large 0 or 2 welding cable. Never had issues for the past 5 years or so until recently.
    Are these Alternators controlled by our ECU at all? What is the single small wire that connects to the alt for? (The 2 pin connector that only has 1 wire)

  2. #2
    Veteran Member Four Rings
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    Nov 02 2014
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    06 9-3 Aero 2.8T, 96 Miata
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    Winchester, VA

    13.5v is acceptable charging voltage.

    The single wire excites the alternator.

    Charging voltage between 13.5v and 14.5v is considered good. If you are getting 13.7, there's no problem.

  3. #3
    Veteran Member Four Rings Bordom's Avatar
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    Dec 10 2013
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    Pickering, Ontario, Canada

    What Diesel said. If your voltage was hovering around the 12.6-13v range, then you have a problem

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  4. #4
    Veteran Member Four Rings MacDaddy's Avatar
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    Yeah its in the range of acceptable but it sounds like there is something going on that is worth looking into incase it turns into a bigger problem.

    I would check the voltage comming out of the reference wire, if its not the same as whats at the battery then theres a possible issue there, not sure if it comes off the ECU but its likeley it does.

    Fwiw i had a land rover that low charging, turned out it was the front window defrost was shorting out and drawing a huge amount of amerage and for some reason the fuse didnt pop, pulled the fuse and charging went back to 14.4

  5. #5
    Veteran Member Four Rings
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    Nov 02 2014
    AZ Member #
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    06 9-3 Aero 2.8T, 96 Miata
    Location
    Winchester, VA

    A bad battery can also draw enough to make charging voltage appear low.

  6. #6
    Veteran Member Three Rings killerkali's Avatar
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    Quote Originally Posted by MacDaddy View Post
    Yeah its in the range of acceptable but it sounds like there is something going on that is worth looking into incase it turns into a bigger problem.

    I would check the voltage comming out of the reference wire, if its not the same as whats at the battery then theres a possible issue there, not sure if it comes off the ECU but its likeley it does.

    Fwiw i had a land rover that low charging, turned out it was the front window defrost was shorting out and drawing a huge amount of amerage and for some reason the fuse didnt pop, pulled the fuse and charging went back to 14.4
    Interesting, from my experience almost every charging system should be in the 14v range, but seems that always isnt the case. I will check the reference wire.

    Not sure if this could indicate anything, but when I activate all the windows "up"(or load the system quickly via other electronics) I get a quick spike to ~12v then it returns. Battery is brand new, have tried my previous battery with same results.

  7. #7
    Veteran Member Four Rings
    Join Date
    Nov 02 2014
    AZ Member #
    292607
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    06 9-3 Aero 2.8T, 96 Miata
    Location
    Winchester, VA

    I work on locomotives now, but I spent 10 years as a line tech for a few name brands. Everywhere was the same standard. 13.5-14.5.

  8. #8
    Veteran Member Four Rings Timtheguru's Avatar
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    Dec 30 2009
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    52711
    Location
    Seattle

    Worn out battery
    2001.5 S4 Sedan
    2002 A4 Avant 1.8t - Traded in
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  9. #9
    Veteran Member Four Rings MacDaddy's Avatar
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    Aug 06 2008
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    Canadia

    Quote Originally Posted by killerkali View Post
    Interesting, from my experience almost every charging system should be in the 14v range, but seems that always isnt the case. I will check the reference wire.

    Not sure if this could indicate anything, but when I activate all the windows "up"(or load the system quickly via other electronics) I get a quick spike to ~12v then it returns. Battery is brand new, have tried my previous battery with same results.
    Yeah thats weird, try pulling he fuse for the windows and see if it changes (also could try other non essential circuits, id probably pull them with the car off to avoid possibility of frying anything) also is it possible to relocate the battery to the stock location temporarily?

    My s4 has always shown just above 14v as long as ive owned it which is almost 10 years now.

    Im not disagreeing with diesel, but with a new alternator and battery things should be higher than .2v above minimum spec.

  10. #10
    Senior Member Three Rings insomniacxp1's Avatar
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    May 10 2010
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    58820
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    01.5 B5 S4
    Location
    Rochester, NY

    ^ I thought about the relocation increasing impedance and effecting the charging voltage but resistance at 10 feet of 0awg is .001 ohm so ....
    01.5 Imola S4. Stage 3 F21's/To much to list

  11. #11
    Established Member Two Rings
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    Apr 25 2015
    AZ Member #
    329432
    Location
    New York

    A bit off topic but is it normal to see any voltage drop after the AC or is turned on?
    For me, sitting in traffic and then turning on the AC, the voltage drop off is noticeable. Nothing to the point of the car dying, just wondering. Thanks

  12. #12
    Veteran Member Four Rings
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    Aug 04 2013
    AZ Member #
    120269
    Location
    Pennsylvania

    Have you verified charging voltage with a multimeter? I've seen inaccurate factory gauges.

    My car's gauge has shown around 13.5 volts since I've owned it, so 5-6 years now.



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